It is estimated that in the United States alone pulmonary embolism causes 200,000 deaths a year. The therapy for suspected or impending pulmonary embolism (PE) is anticoagulation drugs or mechanical interruption of the inferior vena cava (IVA). Both treatments are employed for PE prevention in patients who have had one or more past episodes of PE, or in patients who have never experienced PE but who have evidenced venous clots above the knee level. Both groups usually consist of elderly, debilitated, bedridden patients. Complicated surgery, hip fracture, post-partum, sepsis, and extensive trauma are conditions typically complicated by PE.
Prophylactic anticoagulation is usually established on a chronic basis, requiring frequent clinical and laboratory check-ups. Bleeding from acute (initial hospital therapy) and chronic anticoagulation is common and is recognized as the most common treatment-related complication. The health care cost involved in anticoagulation regimens is accordingly high.
As an alternative to anticoagulation, plain intraoperative ligation of the IVC was first reported in 1944. This procedure was usually followed by a highly incapacitating lower extremity swelling caused by venous stasis and did not always prevent subsequent PE which occurred via collateral veins. In an attempt to avoid such complications, partial IVC occlusions were performed by placing transpiercing sutures by a technique called plication, to divide the caval lumen into multiple small channels. Partial restriction of the lumen was also achieved by use of extraluminal serrated clips or extraluminal prostheses to reduce the luminal cross section to a slot shape or simply by placing staples across the cava to reduce the lumen. Loose metal sutures have also been placed across the lumen in order to create a grid or filter plane without restricting the flow. All such methods required major abdominal surgery in patients who were already in critical condition due to the underlying disease or by pulmonary embolism. Among this group the mortality was high, thus restricting such caval interruption procedures to life threatening situations.
The first attempt to arrest embolization by a less invasive method was made by Eichelter in 1968. Peter Eichelter; "Prophylaxis of Pulmonary Embolism". Arch. Surg. Vol. 97, pp. 348-356 (1968). His "sieve" consisted of a catheter with a non-detachable distal filtering device, placed through the femoral vein and left in lumen of the IVC temporarily. Soon after Eicheleter's publication, several other devices appeared that were also introduced through a peripheral vein such as the femoral or jugular. These devices were all detachable and designed to remain permanently in the lumen of the IVC.
Mobin-Uddin described an umbrella-like device with radial spokes embedded in a perforated plastic disk. Kazi Mobin-Uddin et al; "The Inferior Vena Cava Umbrella Filter". Progress in Cardiovascular Diseases, Vol. 42, No. 5, pp. 391-399 (1975). The Mobin-Uddin device is introduced in a folded fashion inside of a metal capsule and is self-anchoring, after release, by sharp spokes that penetrated the caval wall. A Mobin-Uddin filter has a mandril threaded to the filter to maintain it in straight alignment during delivery. The mandril is unscrewed from the filter after adequate positioning. Experience has demonstrated that Mobin-Uddin umbrella type filters have a high incidence of migration to the heart and causes complete occlusion of the IVC in about half the patients.
In 1969 Pate described a metal clip which was also introduced through a catheter. James W. Pate et al; "A New Form of Vena Cava Interruption". Annals of Surgery, Vol. 169, No. 6, pp. 873-880 (1969). That device caused the caval lumen to become reduced to a slot shaped cross section. In 1970 Hunter described a detachable balloon to produce total occlusion of the IVC. James A. Hunter et al; "Experimental Balloon Obstruction of the Inferior Vena Cava". Annals of Surgery, Vol. 171, No. 2, pp. 315-320 (1970).
Another device, described by Greenfield, consists of a cone shaped array of 6 wires joined at the apex with recurved hooks at the base. Lazar J. Greenfield et al; "A New Intracaval Filter Permitting Continued Flow and Resolution of Emboli". Surgery, Vol. 73, No. 4, pp. 599-606 (1973). Its cartridge design permits trapping of emboli in the apex while allowing continued filtration through the base. The disadvantages of the Greenfield device mainly relate to the progressive separation of the tines toward the base. As the apex fills with clot, larger particles may pass through the base where the spaces are wider; therefore, the filtering ability decreases as the amount of trapped emboli increases. If the Greenfield type filter is deployed in a tilted manner (more than 15 degrees off the axis of the IVC) filtering ability is decreased because the wider spaces between wires is exposed to the central layers of the cava laminar flow which carries the larger emboli. Tilted positioning is not uncommon in this device because it is released by a push rod that ejects the filter out of a capsule. As soon as the tines disengage the capsule they spring open and contact the caval wall. The disengagement may not be simultaneous for all tines, causing the filter to tilt to one side.
All of the remotely introduced filter devices so far described are relatively bulky and have the disadvantage of needing surgical exposure of the jugular or femoral vein in order to be introduced into the cava. It has now become evident that many of the complications related to such filters are due to their misplacement.
Filters made of thermal memory alloys, such as nitinol, have been tried. Although the filter designs varied, the principle of each was the same. Each filter was based on the ability of such an alloy, upon exposure to warm blood, to regain the shape it had prior to annealing. The filter could be introduced in a collapsed fashion through a catheter and would regain its shape in the caval lumen. Unfortunately, the temperature which triggers the change in shape is not critical for such alloys and despite constant infusion of cold saline in the lumen of the introducer tube, premature reshaping often occurs causing jamming of the filter in the catheter.
A different concept was introduced by Roehm. John O.F. Roehm et al; "Percutaneous Transcatheter Filter for the Inferior Vena Cava". Radiology, Vol. 150, No. 1, pp. 255-257 (1984). His filtering device was also introduced percutaneously and was termed a "bird's nest filter." It consisted of four strands of fine steel wire, 25 cm long with four fine hooks (2 strands per hook) at both ends. The strands coiled into a mesh inside of the caval lumen and were fixed in place by the hooks. Several migrations of this filter to the heart have occurred and its filtering ability has been questioned since it has been shown that rather large clots may still pass through the irregular pore sizes of the mesh formed by the wires.
Lund in 1984 reported a device similar to the Greenfield filter but made of thinner wire. Gunnar Lund et al; "A New Vena Caval Filter for Percutaneous Placement and Retrieval; Experimental Study". Radiology, Vol. 152, No. 2, pp. 369-372 (1984). The Lund device had a threaded hub for accurate positioning and a hook at the distal end for eventual removal. The anchoring tines were straight sharp tips with wire loops limiting penetration in the caval wall. The water hammer effect of totally occlusive emboli suddenly meeting an obstruction may dislodge and cause migration of a Lund type filter which is deployed in cephalic projection by femoral vein introduction. Jugular introduction, which would invert the projection, would stabilize a Lund type filter but its filtering ability would be decreased. The second disadvantage of this device is the short span of the tines limit the range of expansion. The caval lumen measures in the average range of 18-22 mm below the renal veins. Nonetheless, 3-12% of the patient population have IVCs larger than 30 mm. Moreover, the caval diameter may change drastically with cough or strain causing straight tines to become dislodged. A large clot suddenly lodging in a filter, enlarges the diameter of the caval lumen at the site of obstruction, pushing the caval wall away from the tines and causing migration of the filter. This is the main cause of migration of the Mobin-Uddin filter and may affect the Lund filter the same way.
Gunther described another filter made of thin wires that can be introduced through a small venous catheter. Rolf W. Gunther et al; "Vena Caval Filter to Prevent Pulmonary Embolism: Experimental Study". Radiology, Vol. 156, No. 2, pp. 315-320 (1985). It consisted of two planes of filtering, a basket and a spider-like portion. The two parts allow it to self-center in the lumen. Its anchoring mechanism is similar to the Lund filter, and therefore subject to the same problems.
Despite the medical community's awareness of the morbidity, high cost and liability involved in anticoagulation as a treatment for the prevention of PE, the alternate treatment -- use of filters -- has not yet gained wide popularity. Most of the physicians arguing against filters base their preference for anticoagulation on the rather large list of complications published for the caval filter designs known to date. Although many of such complications may be avoided in the future by better use of fluoroscopy; nevertheless, migration and ineffective filtering remain as the two primary problems detracting from wider use of known filter devices.
If a vena cava filter having a higher level of acceptance can be developed, not only will the filter be used in life threatening situations (previous life-threatening episode of PE, multiple small episodes of PE leading to progressive occlusion of the pulmonary arteries and pulmonary hypertension, documented clot in large veins) but also in patients without a history of deep venous thrombosis or PE, yet who suffer from any of the following conditions; cancer victims on chemotherapy, particularly if old and/or debilitated, candidates for major surgery and anticipated slow recovery, elderly patients undergoing total hip replacement, complicated postpartum, severe burns, renal vein thrombosis, crushing pelvic fracture or other severe forms of trauma. In such conditions a patient's demise is frequently related to fatal PE, and accounts for most of the estimated 200,000 PE deaths per year which occur in the United States.